Theoretical study on the ring stretching modes of pyridine in the hydrogen bonding with H2O, HCONH2 and CH3COOH

Density functional theory was applied to study the effects of H-bonds N⋯H-X between pyridine and H2O, HCONH2 and CH3COOH on normal vibrational modes of pyridine at the B3LYP/AUG-cc-pVDZ and B3LYP/AUG-cc-pVTZ levels. The results show that the formation of H-bonds leads to an increase in frequencies of the ring breathing mode v1, the N-para-C stretching mode v6a and the meta-CC stretching mode v8a of pyridine but there was no change in triangle mode v12. The natural bond orbital analysis shows that the frequency blue shift in the ring stretching modes of pyridine is a corporate result of the intermolecular charge transfer caused by the intermolecular hyperconjugation n(N) → σ∗(HX) and the intramolecular charge redistribution caused by intramolecular hyperconjugation n(N) → σ∗(meta-CC) in the pyridine ring. We also found that the magnitude of the frequency blue shift increases with the strength of the hydrogen bonding.